Hybrid electric vehicles (HEVs) and Plug-in HEVs (PHEVs) run primarily in an electric mode where the vehicle is propelled by an energy storage device (e.g., a battery). The vehicle may be operated in an engine mode only during selected conditions. As a result, depending on the vehicle drive cycle and the battery charging cycle, the engine may be operated infrequently. This causes fuel to reside in the fuel tank for prolonged durations.
Since fuel can degrade over time, various approaches may be taken to reduce the residence time of fuel in a fuel tank. For example, as elaborated by Leone in US 20100300781, an engine in a hybrid vehicle may be periodically operated to deplete the fuel, even though a system battery is sufficiently charged to operate the vehicle.
However the inventors herein have identified potential issues with such approaches. As one example, even if the engine is periodically started and operated to deplete the fuel, the operator may only partly fill a fuel tank more frequently to reduce what might be viewed as unnecessary engine operation. While this may reduce or avoid fuel degradation, engine oil dilution may be exacerbated. In particular, over the course of vehicle operation, fuel and water may contaminate engine lubricating oils. While at least some of the contaminating water can be removed (e.g., evaporated) during engine warm-ups, in HEVs and PHEVs, where engine run times are infrequent and not sufficiently long, engine temperatures may not be high enough to address the contaminating water. For example, even when the fuel is not stale, oil dilution may occur due to the engine being run with frequency cold starts to improve fuel economy. Even when the engine is periodically started to deplete stale fuel, the engine may not be operated hot enough to remove the contaminating water. Consequently, the diluted engine oil can lead to increased heat, decreased performance, excessive wear, and potential for component failure.
In one example, some of the above issues may be addressed by a method for an engine in a hybrid vehicle comprising: in response to oil dilution being higher than a threshold, operating an engine to raise engine temperature above a threshold temperature for a duration. In this way, an engine may be periodically operated hot so as to reduce oil dilution. At the same time, synergistic benefits in reducing fuel residence time in hybrid vehicles are also achieved.
In one example, a vehicle controller coupled in a hybrid vehicle may monitor each of a residence time of fuel in the fuel tank as well as an engine oil dilution. Engine restarts due to a prolonged fuel residence time may be adjusted based on a history of engine restarts due to stale fuel, and vice versa, so as to provide synergistic benefits. For example, in response to engine oil dilution being more than a threshold amount, an engine may be started even though a battery state of charge is sufficiently high. As an example, if elevated engine oil dilution is detected while the vehicle is in an electric mode, the vehicle may be temporarily and intentionally shifted to the electric mode, even though continued operation in the electric mode was possible and even though engine mode was not requested by a vehicle operator. A duration of engine operation as well as a target engine temperature may be adjusted based on when a last engine oil change was performed as well as based on when the engine was last started to deplete stale fuel. For example, if an engine oil change had occurred recently, the duration of operation may be decreased or the target engine temperature may be decreased. As another example, if the engine was recently started to deplete stale fuel from the fuel tank, the duration and the target engine temperature may be lowered. The duration and target temperature may be further adjusted based on whether the engine start to address the stale fuel was an engine hot start or cold start, with shorter durations and target temperatures applied if the most recent engine start was a hot start.
Likewise, intentional engine starts for the purpose of depleting stagnant fuel may be adjusted based on when the engine was last started for addressing oil dilution. For example, an engine restart due to stagnating fuel may be delayed, or run for a shorter duration if the engine was recently operated to reduce oil dilution. As another example, in response to the presence of old fuel in the fuel tank, the engine may be run with a cold start if oil dilution is lower and run with a hot start if the oil dilution is higher. In still further examples, one or more additional engine parameters may be adjusted differently based on whether the engine is started responsive to oil dilution or started responsive to fuel age. For example, spark timing adjustments may be used during engine starts responsive to oil dilution to assist in raising engine temperatures (e.g., more retarded) while spark timing adjustments may be used during engine starts responsive to fuel age to assist reducing combustion instability (less retarded).
In this way, each of stale fuel and oil dilution can be addressed in hybrid vehicles without running an engine too frequently. By adjusting intentional engine starts to deplete stale fuel based on oil dilution levels, and intentional engine starts to reduce oil dilution based on fuel residence times, an engine start can be used to both reduce water and fuel contamination of engine oil while also reducing fuel degradation, thereby providing synergistic benefits. By operating an engine to raise an engine temperature during some conditions, water and fuel contamination of engine oil can be lowered. By operating the engine to raise fuel usage during other conditions, fuel degradation can be lowered. Overall, hybrid vehicle performance is improved without degrading a vehicle operator's perception of drivability.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.